Method and arrangement for the measurement of the anterior segment of the eye

ABSTRACT

The invention is directed to an arrangement and a method for measuring the anterior segment of the eye using interferometric means. The eye is illuminated by a convergent beam bundle and aligned with the optical axis of the measuring device by generating directional stimuli and accommodation stimuli by means of a display which is mirrored into the beam path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation patent application of U.S. application Ser. No.10/557,566 filed Nov. 21, 2005 now U.S. Pat. No. 7,380,939 which claimspriority of International Application No. PCT/EP04/005170, filed May 14,2004, and German Application No. 103 23 920.0, filed May 22, 2003, thecomplete disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to a method and an arrangement for measuringthe anterior segment of the eye. It is applicable in particular fordetermining the parameters needed for selecting a suitable intraocularlens for a cataract operation. Further, it can also be used for qualitycontrol subsequent to the implantation of an intraocular lens.

b) Description of the Related Art

A device that can be used for contactless measurement of the eye length,corneal curvature and anterior chamber depth is known from DE 198 57001. In this connection, the axial length is determinedinterferometrically, the corneal curvature is determined by means ofimage processing from reflection images of measurement marks that areprojected on the cornea at a determined angle, and the anterior chamberdepth is determined from the evaluation of the back-scattering of acolumn-shaped illumination of the eye length.

The described measurement of the anterior chamber depth does notfunction in the presence of pseudophakia because the implantedintraocular lenses (IOL) generally do not have a scattering effect. Theinterferometric measurement of the axial eye length is known from“Optical Measurement of the Axial Eye Length by Laser DopplerInterferometry” (Ch. Hitzenberger, Investigative Opthalmology and VisualScience, Vol. 32, No. 3, page 616, March 1991), which disclosure isreferred to in the following.

DE 101 08 797 describes a method for determining the diameter of thepupil and iris with digital image processing means, wherein the anglebetween the visual axis and optical axis of the eye, among others, canalso be determined.

A test setup by which the anterior segment of the eye can be measured byinterferometry is described in “Submicrometer Precision Biometry of theAnterior Segment of the Human Eye” (Drexler et al., InvestigativeOpthalmology & Visual Science, Vol. 38, No. 7, page 1304, June 1997).For this purpose, the eye is irradiated by a collimated light bundleduring the measurement process. The light components which are reflectedby the cornea and lens surfaces and are imaged on a photodetector arerelatively weak. The eye must be oriented for measurement in such a waythat its optical axis coincides with the measurement axis of the device.For this purpose, a collimated fixating light is presented to thepatient along a stationary (coaxial) axis, which fixating light iscoupled in by a mirror for the eye to be measured. The adjustment of anangle between the visual axis of the patient and the measurement axis ofthe test setup is carried out by means of a scanning mirror. Even with adeviation of the optical axis from the measurement axis in the range of1° (e.g., due to fixating problems or nystagmus), the reflections of thecornea and lens can no longer overlap so that there is no interferencemeasurement signal. Accordingly, the measurement is very sensitive totilting of the patient's eye. Further, the fixating light always appearsin infinity to the patient, which can be disadvantageous. The positionof the optical axis is found by tilting the scanning mirror in twodirections orthogonal to one another until all measurement signals ofthe cornea and lens are to be detected simultaneously. This method isextremely time-consuming and also does not lead to the desired resultsin all patients. This method is too complicated for routine clinicaluse.

OBJECT AND SUMMARY OF THE INVENTION

The primary object of the invention is to overcome the disadvantages ofthe prior art and to enable a fast and reliable measurement of theanterior segment of the eye by interferometric means.

This object is met, according to the invention, by an arrangement formeasuring the anterior segment of the eye of a patient comprising aninterferometer arrangement is provided for measuring the distances ofoptical functional surfaces of the eye, a light source which is providedfor generating a focus stimulus for the patient and a focusing lenswhich is provided in the beam path of the interferometer. The focusinglens illuminates the eye with a convergent beam bundle. The light sourcefor generating the focusing stimulus is a surface light modulator withadjustable light distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

The invention will be described in the following with reference to thedrawings.

FIG. 1 shows a schematic arrangement of a preferred embodiment example;

FIG. 2 shows a schematic view of a portion of the interferometer beampath;

FIGS. 3 and 4 show two (incomplete) measurement results;

FIG. 5 shows a complete measurement result; and

FIG. 6 shows the transmission characteristic of one of the beamsplittersfrom FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the light emitted by an SLD (superluminescent diode) isguided in direction of the patient's eye (not shown) in a known mannerby an interferometer and a beamsplitter interferometer. A focusing lenswhich focuses the measuring beam in the anterior segment of the eye islocated between the beamsplitter and the eye. The light reflected by theeye is approximately collimated through the focusing lens and is imagedon the photodetector APD (Avalanche Photo Detector), which is locatedbehind a diaphragm, not shown, by means of beamsplitters APD and APDAchromat. A small proportion of the light (about 5%) is transmitted inthe beamsplitter and imaged on a CCD camera by suitable optics(magnification changer).

By deliberate defocusing of the measurement beam by means of thefocusing lens or displacement of the optics relative to the patient'seye along the optical axis of the arrangement, the reflections of thecornea and lens (the front and back, respectively) are imaged out offocus and in magnified form upon the photodetector APD. The magnifiedimaging of the reflections makes it possible to overlap the reflectionsin a large area so as to reduce sensitivity to tilting and lateraldisplacement of the eye. Varying degrees of light intensities of thereflections on the APD are advantageously compensated throughcorresponding adaptation of the electronic amplification. Theamplification can be adapted automatically or reduced by an averageconstant factor.

Further, the sensitivity of the arrangement to deviations of the eyeaxis from the optical axis of the arrangement depends upon the ratio ofthe focal length of the focusing lens to the focal length of the APDAchromat (APD lens). The greater the quotient of the two focal lengths,the lower this sensitivity, so that larger deviations also still delivera signal that can be evaluated.

FIG. 2 shows this dependency of the lateral adjustment accuracy to theeye upon the focal lengths of the lenses that are used. The eye isoffset laterally relative to the device axis (optical axis) by y. Themain beam must pass the APD diaphragm (Ø=2y′) after reflection at thecornea and passage through the focusing lens and APD lens. The APDdiaphragm lies in the focal plane of the APD lens.

The imaging of the virtual object point (height y) in the focal plane ofthe focusing lens is taken into account in order to determine thedependency of the allowable lateral offset upon the focal lengths of thelenses. For the imaging of this imaginary object on the edge of the APDdiaphragm:

${y^{\prime} \approx {\frac{f_{APD}}{f_{FOK}}y\mspace{14mu}{and}}},$therefore, for the lateral offset

${y \approx {\frac{f_{FOK}}{f_{APD}}y^{\prime}}},$where f_(FOK), f_(APD) are the focal lengths of the focusing lens andAPD lens, y is the permissible lateral offset, and y′ is half of thediaphragm diameter.

An optimal value for this ratio f_(FOK):f_(APD) is approximately between1.5 and 4.

In the event that not all of the signals of the cornea and lens can bedetected simultaneously in spite of the enlarged measurement area, aplurality of measurements are carried out and the results of thesemeasurements are suitably combined in a total result containing allnecessary measurement signals. FIGS. 3 to 5 show a possibility for amethod of this kind, wherein ACS is the anterior cornea surface, PCS isthe posterior cornea surface, ALS is the anterior lens surface, and PLSis the posterior lens surface.

No reflection signal of the anterior lens surface (ALS) was detected inFIG. 3. In FIG. 4, the reflection of the posterior lens surface isabsent. In a first step, the two measurement series are transformed to areflection present in both, preferably the reflection of the anteriorcornea surface (ACS). Subsequently, the other values of the reflectionsignals from the two measurements are taken over in the totalmeasurement. Depending on the quality of the measurements and onrequirements, more than two measurements can also be carried out andcombined in the manner described above. It is advantageous when themaximum values associated with the reflections from the differentmeasurements are taken over in the total measurement.

For carrying out measurements, it is advantageous when the optical axisof the measurement arrangement and the optical axis of the eye arealigned to one another. The piercing point of the visual axis throughthe pupil is marked by reflection of a coaxial LED which is arranged inthe optical axis of the measurement arrangement (not shown in FIG. 1 forreasons of simplicity) on which the patient is fixated. The deviation ofthe visual axis from the optical axis of the eye can then be determinedby means of the method described in DE 101 08 797.

An LC display (accommodation stimulus display) which is integrated inthe device is used for the adjustment of the angle required formeasurement and is mirrored into the beam path by an imaging opticsaccommodation and a beamsplitter accommodation. A test mark (e.g.,cross, point, or the like) is automatically displaced as a function ofthis determined deviation in such a way that the patient's eye isoriented for measurement by fixating on this mark. The detectability ofthis LED is decisively improved by the focusing lens, which contributesto increasing the measuring accuracy.

Mirroring the test marks shown in the LC display in the visiblewavelength range into the measurement beam path (infrared) makesspecific demands on the beamsplitter being used (beamsplitteraccommodation). For this purpose, the efficiency of the optical signalto be coupled in from the LC display into the beam path leading to thepatient's eye needs to be as high as possible. The characteristicwhereby, due to its operating principle, the light coming from the LCdisplay is linearly polarized is made use of in order to meet thisdemand. The beamsplitter has a reflection of virtually 100 percent fors-polarized light in the VIS range from 400 nm to 650 nm. At the sametime, lossless transmission, as far as possible, is realized in the nearinfrared range (830 nm . . . 1000 nm).

The layer design meets these requirements for an incident angle rangearound 45°. The materials that are used are matched with one anotherwith respect to the refractive index of the substrate, cement andcoating substances. The following materials were selected for thisspecific use:

Substrate: BK7 n = 1.64 cement n = 1.52 H n = 2.30 L n = 1.48.The design comprises twenty-three alternating layers of H and L.

Corresponding components can be produced for comparable splitters by asuitable selection of the refractive indices of the substrate andcoating substances and the incident angle. The complete layer system canbe displaced by a factor with respect to the edge position.

Parameters: high reflection of 400 nm . . . 660 nm, s-polarized

-   -   high transmission of 830 nm . . . 1000 nm, unpolarized and        s-polarized        Example Data:

1 TiO2 66.28 nm 2 SiO2 91.88 nm 3 TiO2 102.33 nm 4 SiO2 141.87 nm 5 TiO275.32 nm 6 SiO2 156.86 nm 7 TiO2 75.32 nm 8 SiO2 156.86 nm 9 TiO2 75.32nm 10 SiO2 156.86 nm 11 TiO2 77.62 nm 12 SiO2 130.85 nm 13 TiO2 63.37 nm14 SiO2 133.9 nm 15 TiO2 49.88 nm 16 SiO2 113.15 nm 17 TiO2 49.88 nm 18SiO2 113.15 nm 19 TiO2 49.88 nm 20 SiO2 113.15 nm 21 TiO2 64.76 nm 22SiO2 118.7 nm 23 TiO2 41.83 nm

FIG. 6 shows the transmission values for s-polarized and unpolarizedlight which are achieved with this layer construction.

The same display is advantageously used to cause the eye to accommodate.

The imaging optics accommodation images test marks in infinity in adefined position of the display. The patient can view the displayedtests in the non-accommodated state. The imaging of test marks iscarried out at defined distances in front of the patient's eye (e.g., 40cm) by means of (e.g., motor-actuated) displacement of the displayperpendicular to its extension. The patient can only see the testssharply when he or she is accommodated. When a measurement is carriedout in the anterior segment of the eye in different accommodation statesthat can be achieved in this way, the respective distance of theanterior lens and posterior lens from the cornea can be determined sothat the movements or changes in the shape of the eye lens that areresponsible for accommodation can be detected. This can also be used inparticular for monitoring the efficiency of accommodating intraocularlenses.

Further, by evaluating the position of the display in which the patientcan still see sharply, it is possible to determine the accommodationamplitude.

Another advantage of the solution according to the invention is thepossibility of adapting the measurement arrangement to possibledefective vision of the patient by corresponding displacement of the LCdisplay until the patient sees the displayed test marks sharply.

For certain applications (e.g., prior to Lasik OP), it is important tomeasure the cornea thickness at a plurality of points. For this purpose,fixation stimuli (e.g., crosses, points, or the like) are displayed onthe LC display at different predetermined locations. This causes thepatient to change the direction of gaze in a corresponding manner sothat the subsequent measurements of the cornea thickness are carried outat the predetermined point. Predetermined points of this kind can bealong the eye axis but also at a distance of 1.5 mm, 3 mm, and 4.5 mmfrom the axis. It is also possible to realize these target points bymeans of correspondingly arranged discrete LEDs.

The realization of the invention is not limited to the embodimentexample shown herein. Further developments are possible withoutdeparting from the scope of protection.

While the foregoing description and drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges may be made therein without departing from the true spirit andscope of the present invention.

1. An arrangement for measuring distances in the anterior segment of theeye of a patient, comprising: an interferometer arrangement thatmeasures the distances of optical functional surfaces of the eye,including the surfaces of a lens of the eye, wherein the arrangement hasan optical axis and wherein the eye has an optical axis, whereinreflections of at least one of the lens surfaces are detected; a lightsource being provided for generating a focus stimulus for the patient; afocusing lens being provided in the beam path of the interferometer,which focusing lens illuminates the eye with a convergent beam bundle;said light source for generating the focusing stimulus being a surfacelight modulator; and wherein, in measurement of the eye of the patient,means are provided for guiding the optical axis of the eye in apredetermined direction as the patient's eye fixates on a movablefixation mark provided by the light source and focusing lens in order toalign the arrangement optical axis with the eye optical axis.
 2. Thearrangement for measuring distances in the anterior segment of the eyeaccording to claim 1; wherein the surface light modulator is an LCDmatrix or an arrangement of discrete LED.
 3. The arrangement formeasuring distances in the anterior segment of the eye according toclaim 1; wherein the surface light modulator contains a DMD.
 4. Thearrangement for measuring distances in the anterior segment of the eyeaccording to claim 1; wherein means are provided for changing thefocusing of the surface light modulator with respect to the patient'seye.
 5. The arrangement for measuring distances in the anterior segmentof the eye according to claim 1; wherein means are provided fordisplacing optics of the arrangement relative to the patient's eye.
 6. Amethod for measuring distances in the anterior segment of the eye,employing an arrangement according to claim 1, comprising the steps of:carrying out a plurality of interferometric measurements of the opticalinterfaces of the anterior chamber of the eye; and detecting measurementvalues which correspond to one another and calculating a complete set ofmeasurement values by adapting to corresponding values that weremeasured in more than one measurement.
 7. The method for measuringdistances in the anterior segment of the eye according to claim 6;wherein the relative position of the eye axis in relation to the opticalaxis of the arrangement is determined and an alignment of the two axesis brought about by offering a correspondingly adapted fixation stimulusby means of the surface light modulator.
 8. A method for measuringdistances in the anterior segment of the eye, in particular the corneathickness, with an arrangement according to claim 1, comprising the stepof: realizing a measurement at predetermined points of the cornea bygenerating focusing stimuli in a defined manner at predetermined anglesto the optical axis of the interferometer arrangement.
 9. A method formeasuring distances in the accommodation amplitude, employing anarrangement according to claim 4, comprising the step of: detecting thedisplacement range of the surface light modulator in which the patientcan sharply see test marks displayed thereon.
 10. A method for measuringdistances in the anterior segment of the eye, employing an arrangementaccording to claim 4 comprising the step of: displacing the surfacelight modulator in a corresponding manner to adapt to possible defectivevision of the patient.